The other day I was at Starbucks, getting my afternoon pick-me-up, and I was chatting with the barista about her classes at high school. She told me that she was having trouble in math, and that she had to take a basic math course over again to try to pass the standardized test all of the students would have to take later in the year.

“I suck at math!” she exclaimed as she filled my drink cup with ice and espresso.

I was struck by her absolute conviction as a mathematical failure. She made the above statement as confidently and vehemently as one would declare that the Earth is round or the sky is blue. For her, this was her reality and there was no doubting it.

But I doubted it. I saw this young woman several times a week and I knew her to be extremely bright and a hard worker. Working behind the counter at Starbucks while going to school is not easy, and handling often difficult customers while jugging multiple drink orders proved to me that she had a lot more going on than she was willing to give herself credit for. So I called her on what she said.

“I don’t want to hear you talk that way about yourself!” I replied. She stopped what she was doing and looked up at me, with a look of surprise in her face. I don’t think anyone had ever questioned her self-assessment before.

“Do you know what happens when you say stuff like ‘I suck at math’? Your mouth says it, your ears hear it, and then your brain believes it.” I had her attention now, so I kept going.

“I think it’s okay for you to recognize that you are struggling in math right now, and that you have to maybe work extra hard at it. But I don’t want to ever hear you say that again. I want you to say ‘I am good at math,’ or, ‘I’m learning more math everyday’ or something like that, okay?”

I don’t think she entirely bought what I was saying, but I could see that she appreciated the idea.

A few days later, I saw her again. Before I could even tell her my order she exclaimed “Guess what?! I just had a math test and guess what happened? I got an A!”

She went on to explain that although the test was difficult for her, “I kept remembering what you said and I told myself ‘I can do this, I can do this,’ and I stayed calm and I did it!”

I can’t tell you how proud I was of her at that moment. And the best part of it for me was that she got it—she understood that part of succeeding in anything in life is believing that you can, and telling yourself that you are. The power of positive thinking was more than a cliché for her at that moment, it was reality.

A very wise friend of mine once told me that we should never say anything to ourselves that we wouldn’t also say to a dear friend or relative. But how often do we look in the mirror and think “Oh dear God, you look tired today!” or “I feel so fat in these pants,” or “I will never get that bill paid off”? Ever since my friend gave me that advice, I’ve made a conscious effort to ban all negative self-talk. And you know what? It’s really helped me feel better, and accomplish more of my goals.

So here is my challenge for anyone who is reading this article: find your own “I suck at math” statement in your life, and banish it forever. Replace it with something positive and see what happens. I think you’ll be happy with the results!